The invention relates to an extensometer comprising a basic body, at least two tap legs adapted to be pressed against a specimen so as to be frictionally locked thereto, the tap legs being mounted to the specimen such that they are movable relative to each other, a signal transmitter being mechanically coupled to at least one tap leg and providing an output signal representing the amount of a relative movement of the tap legs, and a support device for supporting the basic body and pressing the tap legs against the specimen. Further, the invention relates to a support for an extensometer comprising tap legs.
In material and/or component tests, extension sensors (so-called extensometers) are used to detect the dependence between an external stress of a specimen and the local deformation thereof. Such an extensometer, for example, is described in DE 28 16 444 C3. The known extensometer comprises a basic body on which at least two tap legs are supported. With its tap legs, the extensometer is pressed against the specimen to be tested so as to be frictionally locked thereto. The specimen is deformed by extending, compressing or other external stresses, which leads to a relative movement of the two tap legs. At least one of the two tap legs is coupled to a signal transmitter in the form of an electromechanical transformer. In the case of the extensometer of DE 28 16 444 C3, the transformer is a wire strain gauge strip which is mounted on a leaf spring connecting the two tap legs. The signal transmitter normally used with extensometers, however, may also operate inductively or capacitively. When performing tests with extensometers, it is to be considered that the tap legs are firmly connected to the specimen and frictionally locked thereto. For this purpose, the known extensometer is retained at the specimen itself by clamping elements. Other known extensometers use retaining devices at which the basic body of the extensometer is retained and which firmly press the extensometer with its tap legs against the specimen.
The displacement distances measured by an extensometer amount to from some micometers to some 10 mm. These small displacement distances require highly sensitive and accurately measuring extensometers.
The main problem with the use of extensometers is that the deformation of the specimen due to the external stress is not known in advance. Namely, if the two contact points at which the tap legs abut the specimen do not move along the straight line defined by the two contact points, then the extensometer is slightly rotated via the tap legs during the deformation of the specimen, and the extensometer is thus canted. Due to the canting, the two tap legs can no longer move freely relative to each other, which falsifies the measuring result.